Rough-In: DWV, Water Supply, and Gas Piping Installation

Overview

The rough-in phase is the foundation of every plumbing installation. Before a single fixture is set, the drain-waste-vent (DWV) system, the potable water supply system, and the gas piping branch must all be installed, supported, tested, and documented inside the wall and floor cavities. In this capstone lab you will do exactly that — working from your approved plan set in the full-scale mock-up bay until every pipe, fitting, and valve is in its correct location and every pressure test passes.

DWV Installation

DWV systems carry wastewater away from fixtures and vent sewer gases safely to the atmosphere. Every horizontal drain line must slope continuously toward the building drain. The standard minimum slope for pipes 3 inches and smaller is 1/4 inch of fall per foot of run; pipes 4 inches and larger may use 1/8 inch per foot, though 1/4 inch is preferred and often required. Use a digital level to confirm slope at each hanger location — do not rely on visual judgment alone.

Fittings must be oriented to guide flow in the direction of drainage. Use sanitary tees (not standard tees) for branch connections on horizontal-to-vertical transitions, and long-sweep 90s or 45-degree elbows where horizontal direction changes are necessary. When working with ABS or PVC pipe, apply the manufacturer-approved primer before solvent cement on every joint. Allow adequate cure time before pressurizing the system. Cast-iron no-hub pipe and fittings are joined with stainless-steel shield couplings torqued to the manufacturer's specification with an inch-pound torque wrench.

Hangers and supports must be installed at intervals specified in the applicable plumbing code for each pipe material. Generally, plastic DWV pipe requires support at closer intervals than cast iron. Every vent pipe must be supported at the top of the stack and at intermediate intervals, and must terminate above the roofline at the height required by code and at a safe horizontal distance from any window, door, or air-intake opening.

Once assembly is complete, plug all openings and perform a low-pressure air test witnessed by your instructor. Record the gauge pressure, the start time, the end time, and the result (hold or fail) on the pressure-test log. Zero pressure drop for the full required duration is a passing result.

Potable Water Supply Installation

Water supply piping must be installed to prevent contamination, water hammer, and cross-connections. Begin at the simulated meter location. Run the cold-water main first, then branch off for the hot-water distribution lines.

Install an RPZ (reduced-pressure zone) backflow preventer on the cold-water main as specified in the scenario. An RPZ is a mechanical assembly containing two independently acting check valves and a differential pressure relief valve. It protects the potable supply against both back-siphonage and back-pressure — the highest level of mechanical backflow protection. After installation, complete the ASSE-format backflow test report, documenting each check valve's differential pressure and the relief valve opening point.

Shutoff valves must be installed at locations required by code (typically at each branch serving a fixture group, at each appliance, and at the main). Where a pressure-reducing valve (PRV) is required by the scenario, install it on the cold-water main downstream of the backflow preventer, and set the outlet pressure to the value shown on the plan set.

For copper pipe, use a pipe cutter — not a saw — to produce a square, burr-free cut. Ream and deburr every cut end before assembly. For PEX, use the manufacturer-specified crimp or clamp tool and the correct ring size for the pipe diameter. After all joints and connections are complete, fill the system with water, purge air, and apply the hydrostatic test pressure specified by the applicable code. Hold for the full required duration with zero leaks visible at any joint, valve, or fitting.

Gas Piping Installation

Gas piping demands the highest level of care. You will install a branch of either CSST (corrugated stainless steel tubing) or black-iron pipe from the simulated meter stub to two appliance locations.

For black-iron pipe, use a pipe threader and apply an approved, gas-rated pipe joint compound or PTFE tape rated for gas service to the male threads only. For CSST, follow the manufacturer's fitting instructions precisely — improper CSST fittings are a leading cause of gas leaks.

Before connecting or disconnecting any gas piping, apply lockout/tagout (LOTO) to the simulated gas-meter ball valve: close the valve, attach a hasp, apply your personal lock, and tag the hasp with your name and the date. No one may re-energize the valve until the person who placed the lock removes it.

After assembly, pressure-test with a manometer at the pressure and for the duration required by the applicable fuel-gas code. A manometer measures low pressures in inches of water column, which is the appropriate unit for gas distribution pressure testing. After the pressure hold, inspect every joint with a listed liquid leak-detection solution — bubbling indicates a leak that must be corrected and re-tested. Document gauge readings, start and end times, and results on the gas pressure-test log.

Safety Throughout the Lab

• Wear safety glasses at all times in the mock-up bay; add a face shield when cutting cast iron or soldering.
• Wear gloves when handling solvent cement, primer, or cutting tools.
• Wear knee pads when working at floor level.
• Never use an open flame near gas piping, even for an unrelated task.
• Ventilate the bay when using solvent cement and primer — vapors are flammable and harmful.
• Follow your instructor's direction on compressed-air pressure limits for the air test; over-pressurization of plastic pipe can cause sudden failure.
• Always complete LOTO on the gas meter valve before and after gas work — no exceptions.